The evolving spectrum of the planetary nebula Hen 2-260^⋆

¹ Nicolaus Copernicus Astronomical Center, ul. Rabiańska 8, 87-100 Toruń, Poland
e-mail: Marcin.Hajduk@astri.uni.torun.pl
² Royal Observatory of Belgium, Ringlaan 3, 1180 Brussels, Belgium
³ Centre for Astronomy, Faculty of Physics, Astronomy and Informatics, Nicolaus Copernicus University, Grudziadzka 5, 87-100 Torun, Poland
⁴ Jodrell Bank Centre for Astrophysics, Alan Turing Building, Manchester M13 9PL, UK

Received: 24 September 2013
Accepted: 20 March 2014

Abstract

Aims. We analysed the planetary nebula Hen 2-260 using optical spectroscopy and photometry. We compared our observations with the data from literature to search for evolutionary changes. We also searched for photomertic variability of the central star.

Methods. The object Hen 2-260 was observed with the SAAO 1.0 m telescope (photometry) and the SALT telescope (low resolution spectroscopy). We also used archival high resolution Very Large Telescope spectra and Hubble Space Telescope imaging. The nebular line fluxes were modelled with the Cloudy photoionization code to derive the stellar and nebular parameters.

Results. The planetary nebula shows a complex structure and possibly a bipolar outflow. The nebula is relatively dense and young. The central star is just starting O⁺ ionization (T_eff ≈ 30 000 K). Comparison of our observations with literature data indicates a 50% increase of the [O iii] 5007 Å line flux between 2001 and 2012. We interpret it as the result of the progression of the ionization of O⁺. The central star evolves to higher temperatures at a rate of 45 ± 7 K yr^-1. The heating rate is consistent with a final mass of 0.626^+0.003_-0.005 M_⊙ or 0.645^+0.008_-0.008 M_⊙ for two different sets of post-AGB evolutionary tracks from literature. The photometric monitoring of Hen 2-260 revealed variations on a timescale of hours or days. There is no direct indication for central star binarity in the spectrum nor for a strong stellar wind. The variability may be caused by pulsations of the star.

Conclusions. The temperature evolution of the central star can be traced using spectroscopic observations of the surrounding planetary nebula spanning a timescale of roughly a decade. This allows us to precisely determine the stellar mass, since the pace of the temperature evolution depends critically on the core mass. The method is independent of the absolute age of the nebula. The kinematical age of the nebula is consistent with the age obtained from the evolutionary track. The final mass of the central star is close to the mass distribution peak for central stars of planetary nebulae found in other studies. The object belongs to a group of young central stars of planetary nebulae showing photometric variability.